This invention relates in general to video endoscope aperture wheel drive systems and, in particular, to a drive system for stepping the position of an aperture wheel used in devices for generating video images.
More specifically, but without restriction to the particular embodiment and/or use which is shown and described for purposes of illustration, this invention relates to a drive system for an aperture wheel used in a video processor for generating video images which eliminates oscillation of the aperture wheel when being advanced or stepped to a desired position in accordance with the operation of an aperture wheel stepping motor.
In video image generating systems, such as a video-equipped endoscope or borescope, a light source is utilized for illuminating an object in order to generate a video image. The light source is projected onto the object, and the light reflected from the object is received by a viewing head of a viewing probe which focuses an image upon an imaging device within the probe. Upon receipt of the reflected image, the imaging device converts the image into electrical signals for further processing. For a more detailed description of such imaging systems, reference is made to the disclosures in Robert C. Wheeler, U.S. Pat. No. 4,532,918 and Dominick Danna et al. U.S. Pat. No. 4,539,586 the disclosures of which are hereby incorporated by reference.
The systems disclosed in these above-identified patents, or other such video imaging systems, may utilize a light control wheel in the form of a color filter wheel or a chopper wheel which functions as a shutter between the light source and viewing probe. In those applications in which a color filter wheel is utilized, the color wheel is positioned to selectively rotate a series of different colored filters in the light path to the imaging device to thereby produce a series of color separated images which are sequentially transmitted to the target area of the imaging device. This series of color separated images produces a field sequential color video signal.
In order to generate the sequence of color images, the color filter wheel includes a plurality of color filters circumferentially spaced in an equidistant pattern. The spaces between the color filters do not permit any light transmission, but function as a shutter for preventing light transmission during rotation of the color filter wheel between adjacent filter segments. During the time in which light transmission is blocked, the color separated images received by the imaging device are read out through conventional video processing circuitry. Such a process is disclosed in U.S. Pat. Nos. 4,546,379 and 4,523,224, the disclosures of which are incorporated herein by reference.
In certain applications a color separated imaging system is not necessary nor is it preferred. In such black and white video systems, the light from the light source is transmitted to an object to be viewed, and the light reflected from that object is received by the viewing head of a viewing probe in the manner previously described. In order to enable the video processing circuitry to read the image reflected onto the imaging device within the probe, however, the light must be interrupted in the manner previously described. This light interruption function is affected by the use of a chopper wheel having portions removed which permit the passage of light, with adjacent light blocking or shutter portions enabling the video processing circuitry to read the image projected onto the imaging device during the time the light transmission is interrupted.
When utilizing an imaging system in an endoscope or borescope application, it has been found that a color system has optimum performance within a particular range of distances between the imaging device and the object to be viewed, and a black and white video system has a different optimum performance viewing range. However, in both systems the intensity of the light being received by the video imaging system through the probe must be monitored and controlled for optimum endoscope and/or borescope applications. To this end, an aperture wheel is interposed in the light path between the object to be imaged and the video imaging system in order to control the intensity of the light received from the imaging device. Because the probe may be positioned at different distances from the object being imaged, the intensity of the light reflected onto the probe and transmitted thereby to the video imaging system will vary with the proximity of the imaging device to the object being imaged.
The intensity of the light reflected from the object being viewed can be controlled by the use of different sized apertures, and the interposing of different sized apertures into the light path in response to the proximity of the viewing probe to the object being imaged will produce a substantially uniform light intensity applied to the video imaging system.
The aperture wheel is formed with various sized openings positioned thereon such that rotation of the aperture wheel will sequentially position each of the apertures formed therethrough in optical alignment with the video imaging system. In this manner the intensity of the light reflected onto the imaging device of the probe is controlled by rotation of the aperture wheel, bringing the desired aperture into optical alignment so that the video imaging system has the desired amount of light to be received. As is known to those skilled in the art, the rotational positioning of the aperture wheel may be automatically controlled in response to the proximity of the object being viewed or the intensity of the reflected light.
Because the probe bearing the imaging device is moved relative to the object being viewed, the light reflected from the object into the probe imaging device varies with the proximity of the probe to the object. In order to maintain a pre-determined desirable light intensity on the imaging device probe, the aperture wheel must be rotated for changing the aperture interposed in the light path to the video imaging system in response to these variations in light intensity. Various control systems are known to those skilled in the art for providing a control signal which can be coupled to an aperture wheel drive motor for energizing the motor to rotate or step the aperture wheel in response to changes in the light intensity which, for example, is reflected back from the object being viewed.
Because the probe is moved fairly rapidly to different viewing positions, the aperture wheel must respond quickly in order to control the light intensity to the video imaging system. For this reason the aperture wheel is generally formed from a thin light-weight material which permits rapid movement or indexing of the aperture wheel without creating large inertia forces.
One of the problems associated with rapid movement or stepping of the aperture wheel is oscillation, which is caused by the inertia of the wheel itself, or the inertia of the mechanisms which are used to effect stepping of the aperture wheel in response to movement of the probe. Heretofore, aperture wheel drive systems wherein the aperture wheel was mounted directly on the drive shaft of a stepping motor to effect rotation or stepping of the aperture wheel in either direction, the inertia of the motor armature and the aperture wheel caused oscillation of the aperture wheel after the motor was stopped. Such oscillations degrade the quality of the picture on the video imaging system, causing quite noticeable and undesirable flicker or cyclic increase and decrease in the brightness of the picture. One attempt to eliminate this degradation in picture quality has been the use of a friction clutch on the motor drive shaft. Such an attempt, however, was not satisfactory in that it did not provide consistent results.
The aperture wheel must be accurately positioned to insure that the predetermined aperture is interposed in the optical path in response to the positioning of the probe. The aperture wheel drive system, therefore, must not only eliminate oscillation of the aperture wheel, but accurately control the rotational position of the aperture wheel so that the desired aperture is interposed in the optical path to the video imaging system in response to the distance between the probe imaging device and the object being viewed, or the intensity of the reflected light, so that the system can be coordinated to stop the aperture wheel in the predetermined position.